A conventional integrated circuit contains a plurality of patterns of metal lines separated by inter-wiring spacings, and a plurality of interconnect lines, such as bus lines, bit lines, word lines and logic interconnect lines. Typically, the metal patterns of vertically spaced metallization layers are electrically interconnected by vias. Metal lines formed in trench-like openings typically extend substantially parallel to the semiconductor substrate. Semiconductor devices of such type according to current technology may comprise five or more levels of metallization to satisfy device geometry and micro miniaturization requirements.
A common method for forming metal lines or plugs is known as “damascene”. Generally, this process involves forming an opening (or via) in the dielectric interlayer, which separates the vertically spaced metallization layers. A via is typically formed using conventional lithographic and etching techniques. After a via is formed, the via is filled with copper or copper alloys. Excess metal material on the surface of the dielectric interlayer is then typically removed by chemical mechanical planarization (CMP).
An electrical field exists between two metal lines when there is a voltage potential difference. A high electrical field causes mechanisms such as electro-migration. With time, an extrusion may eventually be formed in the dielectric separating the two metal lines and shorting the metal lines, hence a time dependent dielectric breakdown (TDDB) occurs. The magnitude of the electrical field between metal lines is one of the determining factors of TDDB, and the higher the electrical field is, the more likely a TDDB will occur.
Vias and their neighboring features are typically subject to TDDB. FIG. 1 illustrates an example. Via 2 is connected to a metal line 4, and both are coupled to a high voltage. Metal line 6 is a ground line at 0V. An electrical field distribution is symbolized by arrows 7. The electrical field next to corner 8 is the strongest since corner 8 is typically substantially sharp. FIG. 2 illustrates an extrusion 10 formed due to the strong electrical field shown in FIG. 1. Typically, for a metal line having a substantially rectangular shaped cross section, the extrusion 10 has an angle α close to 45° with respect to a horizontal line A–A′. The extrusion 10 comprising metal material migrated from metal line 6 is formed starting from metal line 6 and extending toward where the electrical field is strongest. With time, the extrusion extends to via 2 or metal line 4. When it touches via 2 or metal line 4, the metal lines are shorted and the circuit fails.
The TDDB problem is more severe for copper vias and copper lines. Copper tends to migrate more readily than other commonly used metals such as tungsten. Copper features are typically used for 0.13 μm and below, therefore are more closely spaced and the electrical fields between copper features are stronger. Also, copper is typically used with low-k inter-metal dielectrics (ILD) that have low material density and porous structures. Therefore, the copper features with low-k materials are more prone to suffer from the time dependent dielectric breakdown problem.
Another problem caused by sharp corners is stress. The sharp corner 8 tends to cause high stresses in the surrounding dielectrics and cause cracks in the passivation layer.
In order to make the electrical field less concentrated at corners and stress reduced, the corners need to be rounded. Typically, when a trench is formed, the corners at the bottom always have a certain degree of curvature. However, the radius of curvature of the naturally formed round corners is not adequate for reducing electrical concentration and stress. Besides, forming round corners using existing processes may introduce other problems. For example, it is observed that narrow-width metal lines typically have higher curvature than wide metal lines formed under the same conditions. Therefore, narrow metal lines are formed where TDDB is likely to occur. However, due to the micro-loading effect, narrow metal lines are thinner than wide metal lines so that they have higher sheet resistance. Therefore, it is not practical to form round corners by reducing the width of the metal lines.